Method and system for decreasing the effective pulse repetition frequency of a laser

ABSTRACT

Methods and systems operate a machine having a laser characterized by a PRF (pulse repetition frequency) parameter that specifies a PRF at which pulses produced by the laser have desirable pulse properties for irradiating structures on or within a workpiece. The structures are arranged on the workpiece in a linear pattern having an approximately equal pitch between adjacent structures. The laser emits a laser pulse that propagates along a laser beam propagation path terminating at a laser beam spot on the workpiece. The method is effective to move the laser beam spot across the structures along the linear pattern at a speed greater than the product of the PRF and the pitch to selectively irradiate selected ones of the structures with the laser without substantially degrading the desirable pulse properties.

RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Application No. 60/599,400, entitled “Method and System forDecreasing the Effective Pulse Repetition Frequency of a Laser,” filedAug. 6, 2004, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

This disclosure relates generally to operation of a pulsed laser andmore particularly to the use of laser beam pulses to process asemiconductor integrated circuit during its manufacturing.

BACKGROUND

During their fabrication process, ICs (integrated circuits) often incurdefects due to minor imperfections in the process or in thesemiconductor substrate. For that reason, IC devices are usuallydesigned to contain redundant circuit elements, such as spare rows andcolumns of memory cells in semiconductor memory devices, e.g., a DRAM(dynamic random access memory), an SRAM (static random access memory),or an embedded memory. Such devices are also designed to includeparticular laser-severable links between electrical contacts of theredundant circuit elements. Such links can be removed, for example, todisconnect a defective memory cell and to substitute a replacementredundant cell. Similar techniques are also used to sever links in orderto program or configure logic products, such as gate arrays or ASICs(application-specific integrated circuits). After an IC has beenfabricated, its circuit elements are tested for defects, and thelocations of defects may be recorded in a data file or defect map.Combined with positional information regarding the layout of the IC andthe location of its circuit elements, a laser-based link processingsystem can be employed to remove selected links so as to make the ICuseful.

A typical link processing system adjusts the position of the laser beamspot on a semiconductor wafer by moving the wafer in an XY planeunderneath a stationary optics table, which supports a laser and otheroptical hardware. The wafer is moved underneath in the XY plane byplacing it on a chuck that is carried by a motion stage. A typical wafercontains a number of dies, each containing an IC. Circuit elementswithin an IC are typically arranged in a regular geometric arrangement,as are the links between those elements. The links usually lie inregular rows in groups that are termed “link banks,” having anapproximately uniform center-to-center pitch spacing. To remove selectedlinks in a link bank, a laser beam spot (i.e., the position at which thelaser beam's propagation path axis intersects the wafer workpiece)continuously advances along the link bank at an approximately uniformspeed while the laser emits pulses to selectively remove links. Thelaser is triggered to emit a pulse and thereby to sever a link at aselected target position when the laser beam spot is on the targetposition. As a result, some of the links are not irradiated and left asunprocessed links, while others are irradiated to become severed. Theprocess of progressing along a row of links and severing selected linkswith a laser pulse is termed a “link run.”

Two of the key parameters that impact the time spent processing a linkrun, and thus throughput, are the maximum velocity limit of the motionstage and the desired link run velocity. Desired link run velocity isthe product of the laser pulse repetition frequency (“PRF”) and the linkpitch. When the desired link run velocity exceeds the maximum velocitylimit of the motion stage, a method must be practiced that results in alink run at a velocity that can be accommodated by the stage. Assume,for example, that the links in a link run have a pitch spacing of 4 μm(micrometers), that the motion stage 170 can travel at a maximum speed(while processing) of 200 mm/sec (millimeters per second), and that thelaser is designed to have an optimum PRF of 50 kHz (kiloHertz). In thatcase, the system can operate at both the optimum PRF and the maximummotion stage speed. As another example, assume that the laser insteadhas an optimum PRF of 60 kHz. In this case, pulsing the laser at thatrate on the links sequentially would require that the motion stage moveat 240 mm/sec, which is faster than it is capable. A simple solution tothis problem is to block every other laser pulse and to slow down themotion stage by a factor of two. The main drawback of that solution isthat it significantly decreases throughput (e.g., link run velocityreduced to 120 mm/sec in this case). Thus, operation of a laser with ahigher PRF can, ironically, slow down the system.

Altering the PRF directly can be disadvantageous, as lasers are oftenoptimized for a particular PRF value. More specifically, the pulsecharacteristics can vary considerably with changes in the PRF as thecharge time of the laser is altered. That is problematic because typicallink processing scenarios require fairly consistent laser pulsecharacteristics from link to link. In other words, the so-called“processing window” for reliably severing links is a small window thatis sensitive to pulse characteristics. Characteristics that may changeas the PRF of a laser is changed include pulse characteristics such astemporal shape, rise time, width, height, energy, energy stability, andbeam propagation characteristics such as beam waist position, beam waistdiameter, and M² value.

U.S. Pat. No. 6,172,325, assigned to the assignee of the presentinvention and incorporated in its entirety herein by reference,describes laser pulse-on-position technology with a fixed laser PRF.Pulse-on-position technology is desirable because it provides veryaccurate placement of link blows; however, that patent does not discussvarying laser PRF.

U.S. Pat. No. 6,339,604, which is also incorporated herein by reference,describes operating a laser with a fixed, predetermined charge time tostabilize pulse properties for the purpose of trimming components on asemiconductor IC. Use of a predetermined time is not possible when usingthe pulse-on-position approach.

SUMMARY

According to one embodiment, a method operates a laser characterized bya PRF parameter that specifies a PRF at which pulses produced by thelaser have desirable pulse properties for irradiating a target on orwithin a workpiece. The laser emits a laser pulse that propagates alonga laser beam propagation path terminating at a laser beam spot on theworkpiece. The method is effective to operate the laser at an effectivePRF lower than the PRF parameter without substantially degrading thedesirable pulse properties. The method receives data indicating thelocation on the workpiece of the target to be selectively irradiatedwith the laser pulse. The method determines a firing position of thelaser beam spot relative to the workpiece at which the laser should emita pulse directed at the target and calculates, based on a desiredcharging time, a charging start position of the laser beam spot relativeto the workpiece at which the laser should begin charging so as tocharge for the desired charging time before the laser beam spot reachesthe firing position. The method moves the workpiece and the laser beamspot relative to one another such that the laser beam spot moves overthe charging start position and then to the firing position and senses aposition of the laser beam spot relative to the workpiece as theworkpiece moves relative to the laser beam spot at a velocity. Themethod commences charging of the laser when the laser beam spot is atthe charging start position, whereby the laser charges for approximatelythe desired charging time, and the method fires the laser when the laserbeam spot reaches the firing position, whereby the laser emits a pulsehaving desired pulse properties. Alternatively, the commencing step canbe based on time rather than position.

According to another embodiment, a method operates a laser having a Qswitch. The laser is characterized by a PRF parameter that specifies aPRF at which pulses produced by the laser have desirable pulseproperties for irradiating targets on or within a workpiece. The laseremits laser pulses that propagate along a laser beam propagation pathhaving a laser beam axis that intersects the workpiece at a laser beamspot when the spot falls on targets on the workpiece. The laser beamspot moves along a surface of the workpiece from target to target in aseries of targets. The method is effective to operate the laser at aneffective PRF lower than the PRF parameter without substantiallydegrading the desirable pulse properties. The method monitors theposition of the laser beam spot relative to the workpiece as the laserbeam spot moves toward a selected target in the series of targets on theworkpiece. The method determines a pre-trigger position at which thelaser beam spot will be located at a approximately constant time beforethe laser beam spot will be located at the selected target position. Themethod opens the Q switch when the laser beam axis is located at thepre-trigger position, triggering the emission of a pulse, and thencloses the Q switch whereby the laser charges for an approximatelyconstant charging time before firing. The method opens the Q switch whenthe laser beam spot is located at the selected target position, wherebythe laser emits a pulse having desired pulse properties to propagatealong the laser beam propagation path and to impinge upon the selectedtarget.

According to another embodiment, a method operates a laser characterizedby an interpulse time parameter that specifies a time between pulsesproduced by the laser and having desirable pulse properties forirradiating a target on or within a workpiece. The laser emits a laserpulse that propagates along a laser beam propagation path terminating ata laser beam spot on the workpiece. The method is effective to operatethe laser at an effective interpulse time greater than the interpulseparameter without substantially degrading the desirable pulseproperties. The method receives data indicating the location on theworkpiece of the target to be selectively irradiated with the laserpulse. The method also determines a firing position of the laser beamspot relative to the workpiece at which the laser should emit a pulsedirected at the target and calculates, based on the interpulse time, apre-pulsing position of the laser beam spot relative to the workpiece atwhich the laser should emit a pulse beginning approximately theinterpulse time before the laser beam spot reaches the firing position.The method moves the workpiece and the laser beam spot relative to oneanother such that the laser beam spot moves over the pre-pulsingposition and then to the firing position and senses a position of thelaser beam spot relative to the workpiece as the workpiece movesrelative to the laser beam spot. The method fires the laser when thelaser beam spot is at the pre-pulsing position, whereby the laser emitsa pre-pulse, and prevents the pre-pulse from reaching the workpiece. Themethod fires the laser when the laser beam spot reaches the firingposition, whereby the laser emits a pulse having desired pulseproperties.

According to yet another embodiment, a system irradiates a selectedtarget in a series of targets on or within a workpiece. The systemcomprises a laser, a laser beam propagation path, a motion stage, aposition sensor, and a controller. The laser has a Q switch and ischaracterized by a PRF parameter that specifies a PRF at which pulsesproduced by the laser have desirable pulse properties. The laser beampropagation path extends from the laser to the workpiece. The path hasan axis that intersects the workpiece at a laser beam spot. The motionstage moves the workpiece and the laser beam spot relative to oneanother. The position sensor indicates the relative position of thelaser beam axis with respect to the workpiece. The controller isconnected to the Q switch and the position sensor. The controlleroperates the laser at an effective PRF lower than the PRF parameterwithout substantially degrading the desirable pulse properties. Thecontroller implements one of the preceding methods.

Details concerning the construction and operation of particularembodiments are set forth in the following sections with reference tothe below-listed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a link processing system according to oneembodiment.

FIG. 2 is a diagram of the laser in the system shown in FIG. 1.

FIG. 3 is a chart showing a laser pulse cycle, interpulse time, chargetime, no-blow zone, and acceptable pre-pulse zone, according to oneembodiment.

FIG. 4 is a chart showing techniques for selecting laser pre-triggeringand link run velocity for the performance of link runs, according tocertain embodiments.

FIG. 5 is a chart showing the states of the Q switch and the AOM ofFIGS. 2 and 1, respectively, during processing of a bank of linksaccording to a technique of FIG. 4.

FIG. 6 is flowchart of a method according to one embodiment.

FIG. 7 is a set of plots of laser beam spot position and laser charge asfunctions of time according to one embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to the above-listed drawings, this section describesparticular embodiments and their detailed construction and operation. Asone skilled in the art will appreciate, certain embodiments may becapable of achieving certain advantages over the known prior art,including some or all of the following: (1) more precise matching of alaser's effective PRF to the requirements of the motion stage duringlink processing operations without adversely affecting laser pulseproperties; (2) higher throughput for link processing in somesituations; (3) greater flexibility to use a laser over a range ofeffective PRFs; (4) more consistent laser pulse properties in caseswhere the PRF is inherently time-varying; and (5) improved systemaccuracy attained through position feedback for pulse triggering. Theseand other advantages of various embodiments will be apparent uponreading the following.

FIG. 1 is a block diagram a link processing system 100 according to oneembodiment. The system 100 comprises a pulsed laser 110, which producesa laser beam 120. The nominal PRF of the laser 110 is the approximatelyconstant desired PRF, which may be selected for a number of reasonsincluding production of desirable pulse characteristics andcompatibility with temporal requirements of the link processing system100. The laser beam 120 propagates along a propagation path until itreaches a workpiece 130 at a laser beam spot 135. Disposed along thepropagation path are a number of optics elements, including anacoustic-optical modulator (AOM) 140, a mirror 150, and a focusing lens160. The AOM 140 is responsive to a radio frequency (RF) input, whichchanges the direction in which the laser beam 120 exits the AOM 140. Byselectively driving the AOM 140 with an RF signal having an appropriateamplitude, the AOM 140 can be configured to selectively block or passthe laser beam 120 to the mirror 150, through the lens 160, and onto theworkpiece 130. In other words, the AOM 140 behaves like a light switchor shutter in the laser beam propagation path. Any device capable offunctioning as a light switch or shutter (e.g., an electro-opticmodulator (EOM) or a liquid crystal modulator) can be used in place ofthe AOM 140.

The workpiece 130 is mounted to a motion stage 170 that moves theworkpiece in an XY plane (the laser beam 120 being incident upon theworkpiece in the Z direction). The firing of the laser 110, theshuttering of the AOM 140, and the movement of the motion stage 170 arecontrolled by a control computer 175, a motion stage controller 180, anda laser controller 185. Together they control laser pulsing and AOMshuttering with the laser beam spot 135 relative to the workpiece 130 bymonitoring the motion stage position, which is commanded by a the motionstage controller 180, and using this position information to coordinatethe laser pulsing and AOM shuttering. Triggering laser emissions basedupon position ensures accurate delivery of laser pulses to target linkstructures. Timing-based laser trigger methods may have diminishedaccuracy due to errors in the XY stage velocity, causing the motionstage 170 to be in an incorrect position when the laser 110 emits apulse.

The control computer 175 accesses a target map (not shown), whichcontains data indicating target positions on the workpiece 130 thatshould be irradiated (e.g., to sever a link at that position). Thetarget map is typically generated, for example, from a testing processthat determines which circuit elements in an IC workpiece are defective.The control computer 175 generates link run commands, sending movementcommands to the motion stage controller 180 and sending laser triggerposition commands to the laser controller 185. The functions of thecontrol computer 175 may be distributed over one or more physicalcomputers.

In addition to affecting movement of the motion stage 170 via controlcommands, the motion stage controller 180 includes a position sensorthat senses where the laser beam spot 135 is relative to the workpiece130. That position data can be used in a feedback control loop toaccurately control the position of the laser beam spot 135 on theworkpiece 130. That measured laser beam spot position data is alsoforwarded to the laser controller 185, where the position data is usedto control the firing of the laser 110. The phrase “laser beam spot” isactually a shorthand expression for the location at which the axis ofthe laser beam's propagation path intersects the workpiece 130, and thesurrounding area of a size and extent approximately equal to what wouldbe illuminated by the laser beam, whether the laser beam is on or off.That spot moves along the top surface of the workpiece 130 as the motionstage 170 moves. Even though the laser beam impinges on that surface attimes (when pulsed and not blocked along its propagation path) and notat other times (when not pulsed or pulsed but blocked), the axis of itspropagation path is always present.

The laser controller 185 comprises two position registers 192 and 194 aswell as a comparator 196. The position register 192 stores the currentposition of the laser beam spot 135 relative to the workpiece 130, whilethe position register 194 stores the position of a target or anotherposition of interest (e.g., a pre-charging or pre-pulsing position) asreported by the control computer 175. The comparator 196 compares thecontents of both position registers 192 and 194 to determine when theymatch. When the comparator 196 indicates a match, the laser controller185 generates commands to the laser 110 and/or the AOM 140, triggeringthe laser to emit a pulse and/or causing the AOM 140 to be set to ablocking or passing state.

Note that it is immaterial whether the laser-optics is stationary andthe workpiece moves (as assumed above for ease of comprehension), orvice versa, or some combination of movement by both bodies occurs. Allthat is required is the laser beam spot 135 and the workpiece 130 moverelative to one another. The purpose of the laser irradiation may be apurpose other than link severing. Laser micromachining, machining,drilling, via drilling, trimming, component trimming, marking, scribing,annealing, changing states of matter, measuring, and reconnecting fusesare a few of the many alternative purposes of the laser irradiation.Nonetheless, link severing is the preferred application. Note finallythat the functions of the control computer 175, motion stage controller180, and laser controller 185 may be performed in fewer physicaldevices, even a single computer or controller.

FIG. 2 is a diagram of an embodiment of the laser 110 in simplifiedform. The laser 110 comprises a laser power supply 204 and RF amplifier206. The laser 110 also comprises two reflectors 210 and 220, whichtogether define a laser cavity 225, which may be a resonator. The laser110 also comprises a pump 230, a laser medium 240, and a Q switch 250.Optionally, the laser resonator 225 may also contain an intracavitysecond harmonic crystal (not shown) and possibly a third harmoniccrystal (not shown). The pump 230 pumps energy into the laser medium240. The pump source 230 preferably provides continuous wave (CW)pumping of the laser medium 240. The laser medium 240 can be, forexample, Nd:YLF, Nd:YAG, or Nd:YVO₄. Other embodiments of the lasermedium 240 may be employed. The pump 230 is preferably a semiconductorlaser. Alternatively, the pump 230 may be a flashlamp or arc lamp orother excitation source suitable for pumping the laser medium 240. Thepump 230 may further include lenses for efficiently coupling the pump230 into the laser medium 240. The action of the pump 230 producesexcited ions suitable for generation of laser output through the processof stimulated emission. The pump 230 may be physically located at theend of the laser cavity 225 (as shown) or to its side. When the Q switch250 is closed, energy is stored in the laser cavity 225 as a result ofthe pumping action. When the Q switch 250 is opened, the stored energypropagates out of the laser cavity 225 as a laser pulse. The opening andclosing of the Q switch 250 is controlled by an RF signal RFQ from theRF amp 206. As those skilled in the art will recognize, FIG. 2 is notmeant to illustrate the physical arrangement of the laser 110accurately.

The laser 110 typically operates in pulsed mode, in which the Q switch250 is alternately closed then opened. During the time when the Q switch250 is closed, energy is stored in the cavity 225, and that energy isreleased as a laser pulse when the Q switch 250 is opened. When thelaser 110 is repetitively Q-switched, the frequency at which the Qswitch 250 is opened is the PRF. As the PRF changes, the properties ofthe laser pulse (e.g., pulse shape, pulse rise time, pulse width, pulseheight, pulse energy, pulse energy stability, etc.) can changesignificantly. For that reason and because link processing systems aretypically highly sensitive to variations in laser pulse properties,lasers in link processing systems typically operate at an approximatelyfixed nominal PRF at which the laser 110 generates pulses havingsuitable properties for link blowing. The selection of the nominal PRFmay also take into account link processing system parameters, such asthe maximum rate at which control computer 175 can process linkcoordinate information.

FIG. 3 is a chart showing different zones of a laser pulse period. Thestart and end of the chart show the generation of two pulses at thenominal laser PRF or period 1/PRF=T_(INTERPULSE). FIG. 3 is not drawn toscale. FIG. 3 depicts the pulse duration, and pulse trigger time, whichis when a pulse begins. FIG. 3 also depicts the binary states of thesignal RFQ in simplified form. The standard laser charging time isT_(CHG)≈T_(INTERPULSE)−T_(EMIT), where T_(EMIT) is the RF off timepre-programmed in the laser power supply 204. FIG. 3 also depicts twozones. A “pre-pulse” zone is shown in which the laser may be pulsedearly by opening the Q switch 250. This results in a depletion of theenergy stored in the laser cavity 225. Pulses emitted when triggeredearly will not have the same pulse properties as a pulse triggered withcharging time T_(CHG). A “no-blow” zone is also depicted. The no-blowzone consists of the duration of the current pulse, plus some additionaltime for system components to change state. System components mayinclude the laser controller 185, the RF amp 204, an AOM switch driver,and the AOM 140. In this time zone, in a preferred embodiment, it is notpossible to trigger laser pulses due to the finite RF off time T_(EMIT),during which the Q-switch 250 remains commanded open. A typical value ofT_(EMIT) is in the range 1–5 μs and can be selected to prevent unwantedparasitic laser pulses from being emitted. Other appropriate values maybe selected.

In another embodiment, the step of pre-pulsing the laser 110 can beomitted, and the laser charge time T_(CHG) can be held approximatelyconstant before each pulse. This method can be practiced by keeping theQ switch 250 open following a pulse until the laser beam spot 135reaches the pre-pulse position or until the pre-pulse time. Then the Qswitch 250 is closed, allowing the laser 110 to charge until triggeredto emit a pulse or dummy pulse. In essence, the pre-pulse positionand/or time is replaced by a laser charge position and/or time. Becausethis method ensures an approximately constant laser charge time T_(CHG),pulse properties are not degraded and the laser 110 is operated at aneffective PRF lower than the nominal PRF.

FIG. 4 is a set of diagrams depicting position-based triggering of laseremissions according to a number of different techniques applied to linkbanks 410, 420, 430, and 440, respectively. FIG. 4A is a diagramdepicting position-based triggering of laser emissions when the link runvelocity, equal to link pitch times nominal laser PRF, is less than orequal to the maximum stage velocity. Link bank 410 comprises a number ofregularly spaced links, the illustrated ones in the segment beinglabeled 450A–459A. In this mode of operation, the laser beam spot 135moves along the link bank 410 left to right as shown. The laser beamspot 135 is shown over the link 459A. Before reaching that position, thelaser beam spot 135 moved across the links 450A–458A in that order. Whenthe position of the laser beam spot 135 coincided with the positions ofeach of the links 450A–458A, the Q switch 250 opened to emit a pulsefrom the laser 110. The AOM 140 was in a blocking state when the laserbeam spot 135 was over the links 452A, 456A, and 458A so that they werenot severed, while the AOM 140 was in a passing state when the laserbeam spot 135 was on the links 450A and 454A so that they were severedby the laser pulse. Because the links 452A, 456A, and 458A were notsevered, they are termed “dummy” targets and the pulses that weredirected toward them but blocked are termed dummy pulses. Likewise, thelaser beam spot 135 will sever the link 459A if the AOM is not in ablocking state when the laser beam spot 135 is on that link.

In FIG. 4A, the time for the laser spot 135 to traverse between targetlinks and dummy links on the wafer is approximately equal to thereciprocal of the nominal PRF; thus the laser interpulse periodT_(INTERPULSE) and charging time T_(CHG) are approximately constant.Minor perturbations in the trigger time can occur because the velocityof the motion stage 170 typically cannot be exactly controlled, and thestage position sensor typically contains some sensor noise. When thestage velocity is slightly higher than the desired link run velocity,laser emission is triggered early and the interpulse period and chargingtime are slightly less than is desired. Likewise, when the stagevelocity is slightly lower than the desired link run velocity, laseremission is triggered late and the interpulse period and charging timeare greater than is desired.

These minor perturbations in charging and trigger times cause minorperturbations in laser pulse properties. However, these minorperturbations in a position-based laser trigger are a tradeoff forhaving laser pulses accurately impinge upon the desired locations oftarget link structures. If laser emission was triggered based upon fixedtime, stage velocity error would lead to improper pulse positioning,with pulses arriving offset from the center of the desired target linksalong the axis of the link run.

In FIG. 4A the spatial distance between adjacent links (e.g., links 452Aand 454A) is denoted d_(INTERPULSE). In other words, the link pitch isd_(INTERPULSE), which is the laser interpulse period T_(INTERPULSE)converted to distance units. For a constant velocity v of the motionstage 170, d_(INTERPULSE) and T_(INTERPULSE) are related by the equationT_(INTERPULSE)=d_(INTERPULSE)/v or the equivalent equationv=d_(INTERPULSE)·PRF. Because FIG. 4A assumes that v can be chosen tosatisfy that equation, the laser 110 can operate at its nominal PRF andtherefore generate pulses having desirable properties for effective linkblowing, as the laser cavity charges for approximately its naturalcharging time t_(CHG) between pulses.

In FIG. 4B, a second mode of operation is depicted that shows theinsertion of pre-pulses. Pre-pulses are added because in this case, thedesired link run velocity v=d*PRF exceeds the maximum permissible linkrun velocity. It is not possible to traverse pitch distance in theinherent time of a laser pulse period. In this case, it is undesirableto diminish throughput by dramatically reducing link run velocity anddelivering every nth pulse to target link structures. It is alsoundesirable to operate the laser at a PRF that is lower than the nominalPRF, since laser pulse properties will be impacted. The laser cavity 225would store more energy than a desirable pulse should have, and pulseproperties such as shape and stability may be degraded.

The mode of operation depicted in FIG. 4B results in link runs at themaximum stage velocity, and very nearly uniform pulse properties. Thismode of operation involves the insertion of pre-pulses, which aretriggered based upon timing or position comparison. Pre-pulses aregenerated to release energy from the laser cavity 225 such that pulses(labeled 530 and 535 on FIG. 5) are substantially equal. By use ofpre-pulses, the laser 110 will have an approximately constant interpulseperiod or distance d_(INTERPULSE), as shown between a pre-pulse position470 and the position of a link 452B. An approximate charging timeresults because the true laser trigger to emit a transmitted pulse to alink or to emit a blocked pulse toward a dummy link is generated byposition comparison. The resultant behavior, is that link runs areperformed at the maximum stage velocity, which is good for throughput,and the laser interpulse period T_(INTERPULSE) or the charge timeT_(CHG) or both are approximately constant, which is good for pulseproperties. Thus, this method reduces the effective laser PRF without anundesirable degradation of pulse properties.

FIG. 4C depicts a third mode of operation, in which the desired link runvelocity is multiple times more than the maximum stage velocity. In thismode, multiple pre-pulses are generated between each actual or dummypulse. By releasing the built-up energy in the laser cavity 225repeatedly in this way, the workpiece 130 can be more effectivelyshielded from unwanted laser radiation than if the laser were allowed tocharge without release up to the point of the final pre-pulse, whichmight then have sufficient energy to leak through the AOM 140 and reachthe workpiece 130.

FIG. 4D depicts a fourth mode of operation, in which the desired linkrun velocity is only slightly more than the maximum stage velocity. Inthis case, the insertion of pre-pulses and completion of the link run atthe maximum velocity is not possible because the pre-pulses would beinserted in the no-blow region depicted in FIG. 3. Instead, in this modethe link run velocity is decreased below the maximum link run velocitysuch that it is possible to use pre-pulses. The minor decrease inthroughput that results is necessary in order to achieve the desiredlaser pulse properties through the use of pre-pulses. However, thisthroughput decrease is minor in comparison with using every other pulseand dividing link run velocity in half. Thus, higher throughput andconsistent pulse properties result.

In all cases illustrated in FIG. 4, triggered laser pre-pulses and dummypulses are blocked from reaching the workpiece 130 by the AOM 140.Desired pulses are allowed to pass through the AOM 140 and reach targetlink structures.

Note also that the position or timing of pre-pulses and regular (blockedor delivered) pulses may also be calculated taking into account the timedelays attributable to hardware in the system 100, e.g., in the laser110, AOM 140, and laser controller 185. Furthermore, it is possible togenerate the pre-pulse and pulse signals to take into account thevelocity error that exists before generation of the pre-pulse. Thismethod involves using the measured or estimated stage velocity from themotion stage controller 180. While this results in a more accurateestimate of what the true laser charging time will be, the actual lasercharging time will still vary due to changes in the stage velocityerror, and noise in the stage position sensor.

Note that although the pitch spacings illustrated in FIG. 4 are uniformin each link bank, that need not be the case. The techniques describedcan be applied naturally to links of non-uniform spacing. In that case,the distance from the previous event-trigger to a current pre-pulseposition may vary while the distance from the current pre-pulse to itsfollowing laser triggering can be set constant by placing the pre-pulseposition appropriately.

FIG. 5 is a chart showing the states of the Q switch 250 and the AOM 140during processing of the link bank 420 depicted in FIG. 4B. The Q switch250 alternates between a closed state 510 and an open state 520. The Qswitch 250 is set in the open state 520 where an actual pulse 530 or adummy pulse 535 is desired as well as where or when a pre-pulse 540 isgenerated. As can be seen, the distance from each pre-pulse 540 to thenext actual or dummy pulse 530 or 535 is the same as d_(INTERPULSE),which results in an interpulse period of approximately T_(INTERPULSE).Consequently, the charging distance d_(CHG) before each dummy pulse 530or actual pulse 535 is approximately constant. The AOM 140 alternatesbetween a passing state 550 and a blocking state 560. The AOM 140 is setin the blocking state 560 during each pre-pulse 540 and during eachdummy pulse 535. However, the AOM 140 is set in the passing state ateach actual pulse 530. Similar charts for the other operation modes ofFIG. 4 can easily be generated using the teachings set forth herein andare therefore omitted.

The foregoing makes clear that the schemes illustrated in FIG. 4 operatethe laser at a lower effective PRF while preserving desirable pulseproperties of the nominal PRF by holding the period of time from thepre-pulse to the actual or dummy pulses approximately constant,consistent with the precise position accuracy of a pulse-on-positionsystem. By adjusting the time that the laser 110 is pre-pulsed, theeffective PRF can be set to precisely match the speed limit of themotion stage 170 and thus maximize throughput.

A numerical example illustrates this advantage. Assume that link pitchis 4 μm and that the maximum speed achievable by the motion stage 170 is200 mm/sec. Assume further that the nominal PRF in FIG. 3 is 50 kHz.Thus, in these circumstances, the scheme in FIG. 3 would be optimum inthe sense that operation of the laser 110 at its nominal PRF exactlymatches the maximum speed achievable by the motion stage 170. However,assume that the laser has a nominal PRF equal to 60 kHz instead (PRF′=60kHz). That means that the optimum interpulse period T_(INTERPULSE)approximately equals 16.67 μsec. However, it takes 20 μsec for the laserbeam spot 135 to traverse the pitch distance d when the motion stagemoves at maximum speed. The schemes illustrated in FIG. 4B can solvethis mismatch by moving the laser beam spot 135 at its maximum speed(200 mm/sec) while pre-pulsing the laser 110 at a time or correspondingposition approximately 3.33 μsec after the previous pulse. This time isapproximately 16.67 μsec before reaching each link and triggering thelaser 110 to emit a pulse. Approximate times result due toposition-based laser pulse generation as the laser beam spot 135 reacheseach link and moves toward the next link.

FIG. 6 is flowchart of a method 600 according to one embodiment. Themethod begins by obtaining (610) link coordinates, which containsposition data for targets and dummy links on the workpiece 130. Thesource of that data may be, for example, a defect map of the workpiece130. The link coordinates are preferably sorted in a sensible order,such as by link runs (i.e., row by row) in order along each link run. Ifnecessary, the method 600 can sort the target data into a desired orderbefore iterating on each target datum in the subsequent steps.

Starting with the first link, the method 600 computes (615) a trigger orfiring position and, if necessary, a pre-pulse position. The method 600moves (620) the laser beam spot 135 relative to the workpiece 130 towardthe pre-pulse position. The method 600 tests (630) whether the sensedposition of the laser beam spot 135 matches the computed pre-pulseposition and continues performing the moving step 620 as necessary untilthey match. When that occurs, the method 600 pre-pulses (635) the laser110 by, e.g., opening the Q switch 250 briefly to release any energybuilt up in the laser cavity 225 while blocking (640) the pre-pulse by,e.g., setting the AOM 140 in a blocking state. Steps 630–640 may berepeated if multiple pre-pulses are required between adjacent links.After the pre-pulse, the method 600 commences (645) charging the laser110 by, e.g., closing the Q switch 250 to terminate the pre-pulse.Meanwhile the method 600 moves (650) the laser beam spot 135 toward thetrigger/link position. In an ideal system, the trigger position is thelink position, but owing to hardware delays, the trigger position may beoffset from the link position slightly. The method 600 tests (655)whether the sensed position of the laser beam spot 135 matches thetrigger position and continues to perform the moving step 650 until theymatch. Then, the method 600 tests (660) whether the link is a dummylink. If so, the method 600 prepares to block (665) the pulse before itreaches the dummy link, e.g., by setting the AOM 140 in a blockingstate. If the link is an actual target to be irradiated, then the method600 skips the blocking step 665, fires (670) the laser 110 by, e.g.,opening the Q switch 250, and ends (675) the laser firing by, e.g.,closing the Q switch 250. Finally, to complete the iteration, the method600 tests (680) whether the current link is the last one. If so, themethod 600 is complete. If not, the method 600 repeats the iterativeloop for the next link by returning to the obtaining step 610. Althoughthe motion of the laser beam spot 135 relative to the workpiece 130could be in discrete steps, it is preferably continuous throughout themethod 600.

FIGS. 7A and 7B are plots of laser beam spot position and laser chargeas functions of time according to one embodiment. In FIG. 7A, theposition of the laser beam spot 135 is plotted as a function of time, asthe laser beam spot 135 moves along a bank of links. The dotted line isa straight line representing the ideal stage velocity, and the solidline represents measured stage velocity. Measured stage velocity hasminor deviations from the ideal stage velocity. The “X” marks along theline represent the estimated position for firing the laser on the links.The diamond marks represent the actual firings. The circle marksrepresent the time/position at which the laser pre-pulse commands areissued an estimated time or distance before each firing position. Thetime difference between each diamond mark and its preceding circle markis approximately the desired interpulse period. The velocity errors andhence timing difference are over-exaggerated on these figures to betterillustrate the difference between estimated and actual laser firingtime.

FIG. 7B shows an approximate representation of the energy in the lasercavity 225 on an identical time scale for an embodiment utilizing aQ-switch laser, such as the laser 110. As can be seen, when the laser110 is fired and the Q switch 250 is closed (at the circle marks) thelaser energy in the cavity 225 builds up until the Q switch 250 isopened (at the diamond marks). Laser cavity energy storage is lowerbefore pre-pulse (circle) events and actual trigger (diamond) events.Careful observation shows that when a pulse occurs early (diamondprecedes X) the laser pulse may have slightly less energy and thesubsequent pre-pulse may have slightly more energy. One again, though,the timing differences between estimated and actual firing time on FIGS.7A and 7B are over-exaggerated, resulting in over-exaggeratedrepresentations of laser cavity energy.

Other embodiments could employ alternative laser technologies. Forexample the laser could be a pulsed fiber laser configured in a masteroscillator power amplifier configuration. In that embodiment, ratherthan relying on a Q-switch to gate the discharge of the laser, thesemiconductor laser employed as the master oscillator to pump the gainfiber in the master oscillator power amplifier configuration can becontrolled in such a fashion as to create the pre-pulse and the actualpulse at the appropriate times. In other alternate embodiments,wavelength-shifted lasers including green lasers or UV lasers, may beemployed.

The methods and systems illustrated and described herein can exist in avariety of forms both active and inactive. For example, they can existas one or more software programs comprised of program instructions insource code, object code, executable code or other formats. Any of theabove can be embodied on a computer-readable medium, which includestorage devices and signals, in compressed or uncompressed form.Exemplary computer-readable storage devices include conventionalcomputer system RAM (random access memory), ROM (read only memory),EPROM (erasable, programmable ROM), EEPROM (electrically erasable,programmable ROM), flash memory and magnetic or optical disks or tapes.Exemplary computer-readable signals, whether modulated using a carrieror not, are signals that a computer system hosting or running a computerprogram can be configured to access, including signals downloadedthrough the Internet or other networks. Concrete examples of theforegoing include distribution of software on a CD ROM or via Internetdownload. In a sense, the Internet itself, as an abstract entity, is acomputer-readable medium. The same is true of computer networks ingeneral.

The terms and descriptions used herein are set forth by way ofillustration only and are not meant as limitations. Those skilled in theart will recognize that many variations can be made to the details ofthe above-described embodiments without departing from the underlyingprinciples of the invention. For example, certain steps of the method600 may be performed in an order other than shown or simultaneously. Thescope of the invention should therefore be determined only by thefollowing claims (and their equivalents) in which all terms are to beunderstood in their broadest reasonable sense unless otherwiseindicated.

1. A method for operating a laser characterized by a PRF parameter thatspecifies a PRF at which pulses produced by the laser have desirablepulse properties for irradiating a target that is one of a set ofstructures on or within a workpiece, the laser emitting a laser pulsethat propagates along a laser beam propagation path terminating at alaser beam spot on the workpiece, the method being effective to operatethe laser at an effective PRF lower than the PRF parameter withoutsubstantially degrading the desirable pulse properties, the methodcomprising: receiving data indicating the location on the workpiece ofthe target to be selectively irradiated with the laser pulse;determining a firing position of the laser beam spot relative to theworkpiece at which the laser should emit a pulse directed at the target;calculating, based on a desired charging time, a charging start positionof the laser beam spot relative to the workpiece at which the lasershould begin charging so as to charge for the desired charging timebefore the laser beam spot reaches the firing position, wherein thecharging start position is not a location of a structure on theworkpiece; moving the workpiece and the laser beam spot relative to oneanother such that the laser beam spot moves over the charging startposition and then to the firing position; sensing a position of thelaser beam spot relative to the workpiece as the workpiece movesrelative to the laser beam spot; commencing charging of the laser whenthe laser beam spot is at the charging start position, whereby the lasercharges for approximately the desired charging time; and firing thelaser when the laser beam spot reaches the firing position, whereby thelaser emits a pulse having desired pulse properties.
 2. The method ofclaim 1, further comprising: pumping the laser continuously at aconstant power.
 3. The method of claim 1, further comprising: repeatingthe determining, calculating, moving, sensing, commencing, and firingsteps for each structure in the set of structures.
 4. The method ofclaim 1, wherein the commencing step comprises: pre-pulsing the laser tothereby release charged laser energy in a pre-pulse and the pre-pulsewhen the laser beam spot is at the charging start position; andpreventing the pre-pulse from reaching the workpiece.
 5. The method ofclaim 1, wherein the laser comprises a Q switch, the commencing stepcomprises closing the Q switch, and the firing step comprises openingthe Q switch.
 6. The method of claim 1, further comprising: preventingthe pulse from reaching the workpiece.
 7. The method of claim 1, furthercomprising: allowing the pulse to reach the workpiece.
 8. The method ofclaim 1, wherein the desired charging time is a function of the PRFparameter.
 9. A method for operating a machine having a lasercharacterized by a PRF parameter that specifies a PRF at which pulsesproduced by the laser have desirable pulse properties for irradiatingstructures on or within a workpiece for a given purpose, the structuresbeing arranged in on the workpiece a linear pattern having anapproximately equal pitch between adjacent structures, the laseremitting a laser pulse that propagates along a laser beam propagationpath terminating at a laser beam spot on the workpiece, the machinebeing capable of moving the workpiece and the laser beam spot relativeto one another at a maximum speed that is less than the product of thePRF parameter and the pitch, the method being effective to move thelaser beam spot across the structures along the linear pattern toselectively irradiate selected ones of the structures with the laserwithout substantially degrading the desirable pulse properties, themethod comprising: receiving data indicating the locations on theworkpiece of the structures; moving the workpiece and the laser beamspot relative to one another such that the laser beam spot moves alongthe linear pattern across the structures at a motion speed less than theproduct of the PRF parameter and the pitch; commencing charging of thelaser approximately a desired charging time before the laser beam spotis expected to reach each structure location, wherein the desiredcharging time is less than the quotient of the pitch divided by themotion speed; firing the laser when the laser beam spot reaches eachstructure location, whereby the laser emits a pulse having desired pulseproperties; and selectively blocking the laser beam propagation pathdepending upon whether the structure to which the laser beam is directedhas been selected for irradiation.
 10. A method according to claim 9,wherein the laser has a Q switch, commencing charging of the lasercomprises closing the Q switch, and firing the laser comprises openingthe Q switch.
 11. The method of claim 9, further comprising: monitoringthe position of the laser beam spot relative to the workpiece as thelaser beam spot; determining charging start positions at which the laserbeam spot will be located at the desired charging time before the laserbeam spot is expected to reach each structure location; and performingthe firing step when the position of the laser beam spot relative to theworkpiece as the laser beam spot matches a charge start position. 12.The method of claim 9, further comprising: calculating times at which toperform the commencing and firing steps; and performing the commencingand firing steps at times calculated by the calculating step.
 13. Themethod of claim 9, wherein commencing charging comprise: pre-pulsing thelaser to emit a pre-pulse so that the pre-pulse ends as the commencingstep begins; and blocking the pre-pulse from reaching the workpiece. 14.The method of claim 9, wherein the workpiece is a semiconductorsubstrate.
 15. The method of claim 14, wherein the structures areelectrically conductive links that are severed when irradiated with thelaser pulse.
 16. The method of claim 9, wherein the desired chargingtime is a function of the PRF parameter.
 17. The method of claim 16,wherein the firing step is initiated in advance of the laser beam spotintersecting each structure location by an amount based on one or moredelays associated with firing the laser.
 18. A computer-readable mediumfor use with a system for operating a machine having a lasercharacterized by a PRF parameter that specifies a PRF at which pulsesproduced by the laser have desirable pulse properties for irradiatingstructures on or within a workpiece for a given purpose, the structuresbeing arranged in on the workpiece a linear pattern having anapproximately equal pitch between adjacent structures, the laseremitting a laser pulse that propagates along a laser beam propagationpath terminating at a laser beam spot on the workpiece, the machinebeing capable of moving the workpiece and the laser beam spot relativeto one another at a maximum speed that is less than the product of thePRF parameter and the pitch, the method being effective to move thelaser beam spot across the structures along the linear pattern toselectively irradiate selected ones of the structures with the laserwithout substantially degrading the desirable pulse properties, thecomputer-readable medium causing the controller to perform a methodcomprising: receiving data indicating the locations on the workpiece ofthe structures; moving the workpiece and the laser beam spot relative toone another such that the laser beam spot moves along the linear patternacross the structures at a motion speed less than the product of the PRFparameter and the pitch; commencing charging of the laser approximatelya desired charging time before the laser beam spot is expected to reacheach structure location, wherein the desired charging time is less thanthe quotient of the pitch divided by the motion speed; firing the laserwhen the laser beam spot reaches each structure location, whereby thelaser emits a pulse having desired pulse properties; and selectivelyblocking the laser beam propagation path depending upon whether thestructure to which the laser beam is directed has been selected forirradiation.